E.g. A character walks across a snow covered field. The polygons creating the snow surface are pushed up by a smooth displacement map. As each foot contacts the surface of the snow, the displacement map influencing the snow surface receives the area of the foot as an update to the displacement map. A foot shaped depression is created in the snow.

A field is covered with Speedtree grass. A mower is driven across the field.
The wheels and the mower blade assembly effect the Speedtree grass in two different ways. The wheels flatten and spread the grass. The blade assembly causes the grass it contacts to instantly shorten creating the effect of cutting the grass and a trail of cut and flattened grass is left behind the mower.



Several multicoloured balls are dropped onto a plane using the physics system. As each ball makes contact with the plane, they leave an area of colour behind, giving the impression that the balls are leaving trails of plant behind.

Last time I saw this type of effect where one object made a localized influence on the shader of a second object was in Lightwave.

These effects would be amazing if possible.

I'm completely unable to answer your question, unfortunately.

I imagine it will hinge on whether displacement map changes can be made along the timeline or whether changes start from frame zero. (Sorry if I should know this from the preview videos).

I like the image you posted. It would be great to see it animated.